This seems to be a common problem on mid 70s Dodges...is there a common problem/cure. I've already changed the blower motor. This happens to be on a 74 truck but I've seen it occur on others also? 20amp fuse gets extremely hot but doesn't blow out?

Check if the fuse holder is rusty at the fuse or around the rivet holding it. Also check the connectors at the switch and under at the heater case. Poor connection leads to high resistance drawing more amperage. Amperage creates heat.They may be melted as well.Doug

This 2820408 A body convertible switch arrived yesterday for restoration and is exhibiting a similar and typical cause of this failure. The center rivet had become slightly loose from the pressure of the outboard circuit breaker movement. Pictures 3 and 4 do show the left to right movement that is occurring.

The effect of this slight arcing between the center connecting blade from the circuit breaker and the rivet, builds up heat and is what takes the hit on the internal return knob centering isolation pin melting. The knob then is unable to apply pressure on the internal swing arm for up or down movement.

The connector thing happened on my '79 van. Turned out to be a bad plug to the heater motor located under the glove box liner. Replaced it and it worked so well I put the original heater motor back in. How a connector in the interior corroded is anyone's guess, but that's electronics for you.

Thanks for the input guys, been laying under the dash checking wires and connections, pulled the glove box liner and was able to disconnect this from the bottom of the melted fuse location, the other connection looks about the same but is part of a "bar" and not removable...could this be it?

As said yes. A HF $9.99 dremel tool/kit works perfect for getting the inside of those tangs to bare metal. In addition there may be poor continuity in the crimp/rivet of that fitting but for a good start I would dremel it & hook it up under the hood with some jumpers from battery to fuse terminal to blower & run it & see if that terminal assy all stays cool by hotwiring the blower for several minutes. EDIT then if all good, reinstall it in the fuse holder box in the pic.

Well, after a couple hours standing on my head...ugh...under the dash looks like a lot of the wiring has been "warm", gotta love these old vehicles, guess I'll just take it wire by wire, plug by plug. The three way switch may be a good call, it's plug in's are cracked and broken and mechanically it isn't very smooth. Thanks for everyone's input, I'll keep plugging along

It's tight but corroded and as a side note this came from the melted side which is also the ground side. May not mean anything but it shows continuity from end to end.

Continuity at zero current is one thing, the ability to pass current is another. The higher the draw (current wise) the more critical connections and contacts become.Those fuse box connections are TOASTED (No pun intended)

Update: thanks to everyone's suggestions and input, disassembled and cleaned every connection I could, including the grounds, put it all back together and now the fan blows on all three speeds without warming the fuse...sweet...but what a pain, lol